Phagocytosis

Phagocytosis is a cellular process in which specialized cells, called phagocytes, engulf and digest large particles such as bacteria, dead cells, and debris. It is a fundamental mechanism of the innate immune system and also links to adaptive immunity through the processing and presentation of antigens.

Professional phagocytes include neutrophils, macrophages, and dendritic cells. Neutrophils are rapid responders that kill ingested microbes, while macrophages and dendritic cells provide sustained pathogen control and can activate T cells after processing and presenting antigens. Phagocytosis can proceed via non-opsonic recognition or through opsonization, where antibodies (such as IgG) or complement components (such as C3b) tag targets to enhance uptake.

The process begins with chemotaxis of phagocytes to sites of infection, followed by attachment of the particle to the cell surface through receptors such as Fc receptors, complement receptors, scavenger receptors, and the mannose receptor. Engulfment forms a phagosome that then matures by acidification and fusion with lysosomes to become a phagolysosome. Within this compartment, lysosomal enzymes and antimicrobial molecules, including reactive oxygen species and reactive nitrogen species, degrade and kill the ingested material. Residual debris may be exocytosed or left as residual bodies, and some phagocytes, particularly dendritic cells, present peptide antigens on MHC class II molecules to helper T cells.

Phagocytosis is regulated by cytokines and pattern-recognition receptor signaling. Interferon-γ and other cytokines activate macrophages, enhancing microbial killing and antigen presentation. Defects in phagocytosis or respiratory burst, as seen in certain immunodeficiencies, can lead to increased susceptibility to infections, illustrating the clinical importance of this process.